A workable (malleable or ductile) composition in metallurgical terms is a metal composition which can be shaped by hot or cold forming in processes other than those involving material removal. While this does not exclude material removal (milling, turning or the like) as possible shaping processes, it is well known that certain metal compositions are incapable of undergoing hot or cold forming (e.g. forging, rolling, pressing and extrusion) in a technologically effective manner and hence resort must be had to such material-removal techniques to shape such metal. Other metal compositions, however, can be rolled, pressed, forged, drawn or extruded and these are generally termed workable or malleable or ductile materials.
With nickle compositions hot-forming techniques are frequently desired and, in addition, cold-forming techniques without material removal may be advantageous.
It is known, for example, that the workability of nickel can be improved by certain procedures conventional in metallurgy. Thus, for example, a nickel melt can be degassed and oxygen can be removed by the formation of carbon monoxide by adding carbon to the melt.
The nickel oxide which is present in the melt is thereby transformed to nickel in accordance with the equation: EQU C + NiO = CO.uparw.+ Ni.
It has been recognized that this equation does not quantitatively represent the elimination of all oxygen from a nickel melt. The equilibrium is dependent upon various parameters such as temperature and slag-forming characteristics so that almost invariably there is a high residue of oxygen in the melt or a return of oxygen to the melt.
In conventional techniques this tendency is counteracted by the addition of oxygen-scavenging elements such as magnesium, aluminum, manganese, silicon and the like, or lithium and titanium, and other elements which have a high affinity for oxygen. It is possible by adding such elements to quantitatively remove oxygen or at least preclude the presence of any nickel oxide.
However the use of these elements as scavengers for oxygen has the disadvantage that the reaction with the oxygen in the melt also can be represented by equilibria and hence the elements with high oxygen affinity must be supplied in considerable excess if an approximately complete deoxidation is to be obtained.
By contrast with the deoxidation with carbon in which the reaction product is removed as a gas, the use of oxygen-scavenging metals results in the presence in the melt and ingot of the corresponding metal oxides. Such deoxidation residues can constitute a handicap in some nickel compositions and may affect the utility of the composition for certain products and forming processes.
For example, such deoxidation residues can, depending upon the thermal conditions during use, diffuse to the surface of the body and interfere with the solderability or weldability of the product. When the nickel composition is to be used to produce parts intended for high-vacuum technology, the application of high vacuum may be detrimentally affected by the presence of such deoxidation residues.
When the nickel bodies are to be coated with glasses or enameled, still another disadvantage ensues. The contact of the deoxidation-metal oxides with elements of the coating material having a high vapor pressure (lead or zinc) results in pore formation. A similar disadvantage results from the presence of carbon residues as may occur when deoxidation is carried out with carbon monoxide formation and carbon remains in the interfacial or boundary layers of the nickel product.
Another disadvantage of nickel materials subjected to conventional deoxidation of the melt and one which is independent of the size of the ingot which is produced, is the tendency of the foreign metal oxides formed during the deoxidation process to precipitate at the grained boundaries and create loci susceptible to high corrosion and high mechanical stress, usually when the nickel bodies are brought into contact with etching alkalies at higher temperatures. Under these conditions, stress-crack corrosion develops as a major disadvantage.